Efavirenz is a well known non-nucleoside reverse transcriptase inhibitor (NNRTI) that is used in the antiretroviral therapy of HIV-1. Efavirenz is on the market for this use since 1998.
Several processes are known in the prior art to produce this active substance that start from the intermediate of formula (II):

This intermediate, (2S)-2-(2-amino-5-chlorophenyl)-4-cyclopropyl-1,1,1-trifluorobut-3-yn-2-ol, as a free base, is reacted with a chloroformate derivative in basic conditions to give, after 1 to 6 hours reaction, an alkyl carbamate intermediate (reaction of the amine moiety of intermediate (II) with the chloroformate derivative) that is then cyclized to yield Efavirenz.
Even if this kind of reaction allows to obtain Efavirenz with an acceptable impurities profile, it has some disadvantages due to the length of the reaction time and/or the quite complicate work-up, as the alkyl carbamate intermediate must be isolated or the reaction mixture must be at least separated and concentrated before undergoing the cyclization step.
No one-step process using chloroformate derivatives has been proposed so far, nor thrichloromethylchloroformate (diphosgene) has been proposed as a reactant of choice in this process.